Fog is formed by the condensation of water vapour on condensation nuclei that are always present in natural air . This results as soon as the relative humidity of the air exceeds saturation by a fraction of 1 percent.
In highly polluted air the nuclei may grow sufficiently to cause fog at humidities of 95 percent or less. Growth of the drops may be helped by the absorption of certain soluble gases, notably sulfur dioxide to form dilute sulfuric acid. The relative humidity of the air can be increased by three processes: cooling of the air by adiabatic expansion; mixing two humid airstreams having different temperatures; and direct cooling of the air by radiation…..
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